Flukes belong to subclass Digenea, class Trematoda in the phylum Platyhelminthes. These digeneans all have an intermediate host and are found exclusively in vertebrates, including man. Families which have members of considerable veterinary importance are Fasciolidae, Dicrocoeliidae, Paramphistomatidae, and Schistosomatidae.
The adult trematode flukes occur primarily in the bile ducts, alimentary tract, and vascular system. Most flukes are flattened dorso-ventrally, have a blind alimentary tract, suckers for attachment, and are hermaphroditic. Depending on the predilection site, the eggs pass out of the final host, usually in faeces or urine, and the larval stages develop in a molluscan intermediate host. For a few species, a second intermediate host is involved, but the mollusc is essential for all members of the group. They are worldwide in their distribution.
Several clinical syndromes may be associated with liver fluke (Fasciola sp.) infection, depending on the numbers and stage of development of the parasite and on the presence or absence of certain bacteria (Clostridium novyi). Acute fluke disease occurs during invasion of the liver by recently ingested metacercariae. In heavy invasions, the trauma inflicted by the maritas tunneling about in the liver and consequent inflammatory reaction result in highly fatal clinical illness characterized by abdominal pain with a disinclination to move. Sheep can die very quickly due to focal liver necrosis and extensive subcutaneous hemorrhage.
Subacute and chronic fluke disease is associated with the presence of adult trematodes in the bile ducts and characterized by the classical clinical signs of liver fluke infection. There is gradual loss of condition, progressive weakness, anemia, and hypoproteinemia with development of edematous subcutaneous swellings, especially in the intermandibular space and over the abdomen. There is considerable economic waste in cattle livers condemned as unfit for human consumption; destructive migrations in the livers of sheep and goats virtually preclude small ruminant production in endemic areas.
Light infections of flukes may not elicit clinical effects, but the parasites can have a significant effect on production due to an impairment of appetite and to their effect on post-absorptive metabolism of protein, carbohydrates, and minerals.
Dicrocoelium dendriticum, a fluke in the family Dicrocoeliidae, of economic importance in sheep, cattle, and pigs, has a life cycle adapted to a sequence of hosts that frequent dry habitats. Adults are parasites of the bile ducts of their hosts and have life cycles utilizing snails and ants as their intermediate hosts. Although clinical illness is absent in young animals, these trematodes are long-lived and the pathological changes in the liver increase in severity and extent with the duration of the infection. Ergo, in older sheep, D. dendriticum infection causes progressive hepatic cirrhosis manifested clinically as cachexia, lowered wool production, decreased lactation, and premature aging. Essentially, it makes sheep husbandry unprofitable by curtailing the reproductive life of the ewe flock. Platynosomum fastosum, another member of this family, is a parasite of the bile and pancreatic ducts of cats, occurring in the southern U.S. and the Caribbean; infection is acquired by eating lizards containing metacercariae.
Control of fascioliasis may be approached in two ways: by reducing populations of the intermediate snail host or by using anthelmintics.
Theoretically, aquatic snails can be controlled by draining swamps or by broadcasting molluscicides on the snail-infested waters, but the continued existence of flukes where they have always been indicates that snail control measures are impracticable in many cases. Areas connected by streams with other snail-infested regions are generally not amenable to snail control measures.
Anthelmintic medication in the U.S. currently consists primarily of albendazole, which is available in only a few states where Fasciola hepatica poses a serious problem. Special dispensation is required to treat sheep with albendazole elsewhere in the U.S. Other effective flukicides--diamphenethide, nitroxynil, oxyclozanide, rafoxanide, and triclabendazole--are not available in the U.S.
Regular use of chemicals to control unwanted organisms can select for drug resistant strains. This has occurred in many species of economically important pests. The development of drug resistance necessitates a continuing search for new control agents having different modes of action.
The bacterium Bacillus thuringiensis (B.t.) produces .delta.-endotoxin polypeptides that have been shown to have activity against a number of insect species. These toxins are deposited as crystalline inclusions within the organism. Many strains of B.t. produce crystalline inclusions with no demonstrated toxicit to any insect tested.
A small number of research articles have been published about the effects of .delta.-endotoxins from B. thuringiensis species on the viability of nematode eggs. Bottjer, Bone, and Gill (Experimental Parasitology 60:239-244, 1985) have reported that B.t. kurstaki and B.t. israelensis were toxic in vitro to eggs of the nematode Trichostrongylus colubriformis. In addition, 28 other B.t. strains were tested with widely variable toxicities. Ignoffo and Dropkin (Ignoffo, C. M. and Dropkin, V. H. [1977] J. Kans. Entomol. Soc. 50:394-398) have reported that the thermostable toxin from Bacillus thuringiensis (beta exotoxin) was active against certain nematodes. Beta exotoxin is a generalized cytotoxic agent with little or no specificity. Also, H. Ciordia and W. E. Bizzell (Jour. of Parasitology 47:41 [abstract]1961) gave a preliminary report on the effects of B. thuringiensis on some cattle nematodes.
At the present time there is a need to have more effective means to control liver flukes that cause considerable damage to susceptible hosts. Advantageously, such effective means would employ biological agents.